DC converter

ABSTRACT

A primary winding  5   a  of a transformer T and a switch Q 1  are connected in series with a DC power source Vdc 1 . A series circuit composed of a switch Q 2  and a snubber capacitor C 3  is connected in parallel with the primary winding or the switch Q 1 . A saturable reactor SL 1  is connected in parallel with the primary winding 5 a . A series circuit composed of a diode D 1  and a capacitor C 4  is connected in parallel with a secondary winding  5   b  of the transformer T, to form a rectifying/smoothing circuit. A control circuit  10  turns on and off the switches Q 1  and Q 2  alternately and turns off the switch Q 2  when a current to the switch Q 2  increases.

TECHNICAL FIELD

The present invention relates to a high-efficiency, small, low-noise DCconverter.

BACKGROUND TECHNOLOGY

FIG. 1 is a circuit diagram showing a DC converter of this typeaccording to a related art. In the DC converter shown in FIG. 1, a DCpower source Vdc1 is connected through a primary winding 5 a (the numberof turns being n1) of a transformer T to a main switch Q1 which may be aMOSFET (hereinafter referred to as FET). Both ends of the primarywinding 5 a are connected to a parallel circuit, which is composed of aresistor R2 and a snubber capacitor C2, and a diode D3 connected inseries with the parallel circuit. The main switch Q1 is turned on andoff under PWM control by a control circuit 100.

The primary winding 5 a and a secondary winding 5 b of the transformer Tare wound so as to generate in-phase voltages. The secondary winding 5 b(the number of turns being n2) of the transformer T is connected to arectifying/smoothing circuit composed of diodes D1 and D2, a reactor L1,and a capacitor C4. The rectifying/smoothing circuit rectifies andsmoothes a voltage (on/off-controlled pulse voltage) induced on thesecondary winding 5 b of the transformer T and provides a DC output fora load RL.

The control circuit 100 has an operational amplifier (not shown) and aphotocoupler (not shown). The operational amplifier compares an outputvoltage of the load RL with a reference voltage. If the output voltageof the load RL is equal to or greater than the reference voltage, theON-width of a pulse applied to the main switch Q1 is controlled to benarrower. Namely, when the output voltage of the load RL becomes equalto or greater than the reference voltage, the ON-width of a pulseapplied to the main switch Q1 is narrowed to maintain a constant outputvoltage.

Operation of the DC converter with the above-mentioned configurationwill be explained with reference to a timing chart shown in FIG. 2. InFIG. 2, the main switch Q1 has a terminal voltage Q1 v, passes a currentQ1 i, and is on/off-controlled according to a Q1-control signal.

At time t31, the Q1-control signal turns on the main switch Q1, and theDC power source Vdc1 passes the current Q1 i through the primary winding5 a of the transformer T to the main switch Q1. This current linearlyincreases as time passes up to time t32. The primary winding 5 a passesa current n1 i that linearly increases as time passes up to time t32,like the current Q1 i.

Between time t31 and time t32, the primary winding 5 a is negative onthe main switch Q1 side, and the primary winding 5 a and secondarywinding 5 b are in-phase. Accordingly, an anode of the diode D1 ispositive to pass a current in order of 5 b, D1, L1, C4, and 5 b.

Next, at time t32, the main switch Q1 is changed from the ON state to anOFF state according to the Q1-control signal. At this time, amonginduced energy on the primary winding 5 a of the transformer T, theinduced energy of a leakage inductance Lg (inductance not coupled withthe secondary winding 5 b) is not transferred to the secondary winding 5b and is accumulated in the snubber capacitor C2 through the diode D3.

Between time t32 and time t33, the main switch Q1 is OFF, and therefore,the current Q1 i and the current n1 i passing through the primarywinding 5 a become zero. Between time t32 and time t33, a current ispassed in order of L1, C4, D2, and L1 to supply power to the load RL.

This DC converter inserts the snubber circuit (C2, R2) to relax atemporal change in the voltage of the main switch Q1, thereby reducingswitching noise and suppressing a surge voltage from the leakageinductance Lg of the transformer T to the main switch Q1.

DISCLOSURE OF INVENTION

The DC converter of FIG. 1, however, increases a loss because chargeaccumulated in the snubber capacitor C2 is consumed by the resistor R2.This loss is proportional to a conversion frequency. If the conversionfrequency is increased to reduce the size of the DC converter, the lossincreases and the efficiency decreases.

A transformer excitation current to the primary winding 5 a of thetransformer T linearly positively increases as shown in FIG. 4 when themain switch Q1 is ON and linearly decreases to zero when the main switchQ1 is OFF. Namely, magnetic flux of the transformer T uses only thefirst quadrant (ΔB′) of a B-H curve, to decrease the usage rate of acore of the transformer T and increase the size of the transformer T.

The present invention provides a DC converter that is capable ofreducing the size of a transformer and realizing zero-voltage switchingand is small, highly efficient, and low in noise.

A first technical aspect of the present invention provides a DCconverter including a first series circuit connected in parallel with aDC power source and involving a primary winding of a transformer and afirst switch that are connected in series, a saturable reactor connectedin parallel with the primary winding of the transformer, a first returncircuit connected to the first series circuit and composed of a secondswitch and a snubber capacitor that are connected in series, to returnenergy accumulated in the saturable reactor, a rectifying/smoothingcircuit connected in parallel with a secondary winding of thetransformer and composed of a rectifying element and a smoothingelement, and a control circuit to turn on and off the first and secondswitches alternately.

According to a second technical aspect of the present invention, the DCconverter of the first technical aspect further includes a power supplysource to accumulate power when the first switch is ON and supply thepower to the snubber capacitor when the first switch is OFF. The firstreturn circuit is connected in parallel with any one of the first switchand primary winding. The control circuit turns off the second switchwhen a current to the second switch increases.

According to a third technical aspect of the present invention, the DCconverter of the first technical aspect connects the first switch of thefirst series circuit through a third reactor to the primary winding andfurther includes a second return circuit connected to the transformer,to return energy accumulated in the third reactor to the secondary sideof the transformer.

According to a fourth technical aspect of the present invention, the DCconverter of the first technical aspect further includes a power supplysource to accumulate power when the first switch is ON and supplies thepower to the snubber capacitor when the first switch is OFF, connectsthe first return circuit in parallel with any one of the first switchand primary winding, and includes in the rectifying/smoothing circuit asecond rectifying element connected in parallel with the secondarywinding of the transformer through the rectifying element and a fourthreactor connected between the rectifying element and the smoothingelement. The control circuit turns off the second switch when a currentto the second switch increases.

According to a fifth technical aspect of the present invention, the DCconverter of the first technical aspect includes a power supply sourceto accumulate power when the first switch is ON and supplies the powerto the snubber capacitor when the first switch is OFF, connects thefirst return circuit in parallel with any one of the first switch andprimary winding, and includes in the rectifying/smoothing circuit afourth reactor connected between the smoothing element and the secondarywinding of the transformer, a third switch connected in parallel withthe rectifying element and having a control terminal connected to asecond end of the secondary winding, a fourth switch connected inparallel with the series circuit of the third switch and secondarywinding and having a control terminal connected to a first end of thesecondary winding, and a second rectifying element connected in parallelwith the secondary winding of the transformer through the third switch.The control circuit turns off the second switch when a current to thesecond switch increases.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram showing a DC converter according to arelated art;

FIG. 2 is a timing chart showing signals at various parts of the DCconverter of the related art;

FIG. 3 is a view showing the B-H characteristic of a transformerarranged in the DC converter of the related art;

FIG. 4 is a timing chart showing an excitation current to thetransformer arranged in the DC converter of the related art;

FIG. 5 is a circuit diagram showing a DC converter according to a firstembodiment;

FIG. 6 is a structural view showing a transformer arranged in the DCconverter according to the first embodiment;

FIG. 7 is a timing chart showing signals at various parts of the DCconverter according to the first embodiment;

FIG. 8 is a timing chart showing signals at various parts of the DCconverter according to the first embodiment when a switch Q1 is turnedon;

FIG. 9 is a view showing the B-H characteristic of the transformerarranged in the DC converter according to the first embodiment;

FIG. 10 is a timing chart showing a current to a saturable reactorarranged in the DC converter according to the first embodiment;

FIG. 11 is a circuit diagram showing a first example of the DC converteraccording to the first embodiment;

FIG. 12 is a circuit diagram showing a first modification of the DCconverter according to the first embodiment;

FIG. 13 is a circuit diagram showing a second modification of the DCconverter according to the first embodiment;

FIG. 14 is a circuit diagram showing a DC converter according to asecond embodiment;

FIG. 15 is a circuit diagram showing a DC converter according to a thirdembodiment;

FIG. 16 is a view explaining operation of the DC converter according tothe third embodiment;

FIG. 17 is a timing chart showing signals at various parts of the DCconverter according to the third embodiment;

FIG. 18 is a timing chart showing signals at various parts of the DCconverter according to any one of the first and second embodiments witha high input voltage;

FIG. 19 is a circuit diagram showing a DC converter according to afourth embodiment;

FIG. 20 is a timing chart showing signals at various parts of the DCconverter according to the fourth embodiment;

FIG. 21 is a timing chart showing signals at various parts of the DCconverter according to the fourth embodiment when a switch Q1 is turnedon;

FIG. 22 is a circuit diagram showing a first modification of the DCconverter according to the fourth embodiment;

FIG. 23 is a circuit diagram showing a second modification of the DCconverter according to the fourth embodiment;

FIG. 24 is a circuit diagram showing a DC converter according to a fifthembodiment;

FIG. 25 is a structural view showing a transformer arranged in the DCconverter according to the third and fifth embodiments;

FIG. 26 is a circuit diagram showing a DC converter according to a sixthembodiment;

FIG. 27 is a timing chart showing signals at various parts of the DCconverter according to the sixth embodiment;

FIG. 28 is a circuit diagram showing a DC converter according to aseventh embodiment;

FIG. 29 is a timing chart showing signals at various parts of the DCconverter according to the seventh embodiment;

FIG. 30 is a circuit diagram showing a DC converter according to aneighth embodiment;

FIG. 31 is a timing chart showing signals at various parts of the DCconverter according to the eighth embodiment;

FIG. 32 is a timing chart showing signals at various parts of the DCconverter according to the eighth embodiment when a switch Q1 is turnedon;

FIG. 33 is a circuit diagram showing a first example of the DC converteraccording to the eighth embodiment;

FIG. 34 is a circuit diagram showing a first modification of the DCconverter according to the eighth embodiment;

FIG. 35 is a circuit diagram showing a second modification of the DCconverter according to the eighth embodiment;

FIG. 36 is a circuit diagram showing a DC converter according to a ninthembodiment; and

FIG. 37 is a timing chart showing signals at various part of the DCconverter according to the ninth embodiment.

BEST MODE OF IMPLEMENTATION

DC converters according to embodiments of the present invention will beexplained in detail with reference to the drawings.

First Embodiment

A DC converter according to a first embodiment directly supplies powerto a load from a secondary winding of a transformer when a main switchis turned on and accumulates induced energy accumulated in a primarywinding of the transformer when the main switch is turned off. When anauxiliary switch is turned on, the first and third quadrants of the B-Hcurve of a core of the transformer are used, and a shortage ofexcitation energy is supplemented from a power supply source so that thestart of the B-H curve is brought to a lower end of the third quadrant.The primary winding of the transformer is connected in parallel with asaturable reactor. The saturable reactor is saturated just before theend of an ON period of the auxiliary switch, to increase a current andsteeply generate a reverse voltage when the auxiliary switch is turnedoff. This results in making the main switch perform a zero-voltageswitching operation.

The secondary side of the transformer T is provided with arectifying/smoothing circuit composed of a diode D1 and a capacitor C4.

FIG. 5 is a circuit diagram showing the DC converter according to thefirst embodiment. In the DC converter shown in FIG. 5, both ends of a Dcpower source Vdc1 are connected to a series circuit (first seriescircuit) including a primary winding 5 a (the number of turns being n1)of the transformer T and a switch Q1 (first switch) made of a FET andserving as a main switch. Both ends of the switch Q1 are connected inparallel with a diode D3 and a resonant capacitor C1.

A node between a first end of the primary winding 5 a of the transformerT and a first end of the switch Q1 is connected to a first end of aswitch Q2 (second switch) made of a FET and serving as an auxiliaryswitch. A second end of the switch Q2 is connected through a snubbercapacitor C3 to a positive electrode of the DC power source Vdc1. Aseries circuit composed of the snubber capacitor C3 and switch Q2 formsa first return circuit. The second end of the switch Q2 may be connectedthrough the snubber capacitor C3 to a negative electrode of the DC powersource Vdc1.

Both ends of the snubber capacitor C3 are connected to a power supplysource Idc1 made of a current source that accumulates power when theswitch Q1 is ON and supplies the accumulated power to the snubbercapacitor C3 when the switch Q1 is OFF.

Both ends of the switch Q2 are connected in parallel with a diode D4.The switches Q1 and Q2 are alternately turned on and off under PWMcontrol by a control circuit 10 and have a period (dead time) in whichboth of them are OFF.

The primary winding 5 a of the transformer T is connected in parallelwith a saturable reactor SL1. The saturable reactor SL1 employs thesaturation characteristic of a core of the transformer T. The saturablereactor SL1 passes an AC current with equal positive and negativeamplitudes. Accordingly, magnetic flux increases and decreases equallyin the first and third quadrants around an origin (B, H)=(0, 0) on a B-Hcurve shown in FIG. 9.

Since the circuit involves a loss, the magnetic flux is not completelysymmetrical, and the first quadrant mainly works. To quickly dischargethe capacitor C1 to realize a zero voltage, the saturable reactor SL1 orthe excitation inductance of the transformer T is lowered to increase anexcitation current.

As shown in FIG. 9, magnetic flux B saturates at Bm with respect to agiven positive magnetic field H and at −Bm with respect to a givennegative magnetic field H. A magnetic field H occurs in proportion tothe magnitude of a current i. Accurately, “B” is a magnetic fluxdensity, and magnetic flux φ is expressed as φ=B·S where S is asectional area of the core. For the convenience of explanation, S=1 andφ=B. In the saturable reactor SL1, magnetic flux B moves along the B-Hcurve in order of Ba, Bb, Bc, Bd, Be, Bf, and Bg. Namely, the operationrange of the magnetic flux is wide. A section Ba-Bb and a section Bf-Bgon the B-H curve are in a saturated state.

Around the core of the transformer T, there are wound the primarywinding 5 a and a secondary winding 5 b (the number of turns being n2)whose phase is the same as that of the primary winding. A first end ofthe secondary winding 5 b is connected to an anode of the diode D1. Acathode of the diode D1 and a second end of the secondary winding 5 bare connected to the capacitor C4. The diode D1 and capacitor C4 formthe rectifying/smoothing circuit. The capacitor C4 smoothes a rectifiedvoltage of the diode D1 and provides a DC output to the load RL.

The control circuit 10 turns on and off the switches Q1 and Q2alternately. The control circuit 10 narrows the ON-width of a pulseapplied to the switch Q1 and widens the ON-width of a pulse applied tothe switch Q2 when the output voltage of the load RL is equal to orabove a reference voltage. Namely, when the output voltage of the loadRL becomes equal to or greater than the reference voltage, the ON-widthof a pulse to the switch Q1 is narrowed to maintain a constant outputvoltage.

As shown in FIG. 8, the control circuit 10 turns off the switch Q2 whena current Q2 i to the switch Q2 increases, and then, turns on the switchQ1. The control circuit 10 turns on the switch Q1 within a predeterminedperiod after the time when the voltage of the switch Q1 becomes zero dueto resonance between the resonant capacitor C1, which is connected inparallel with the switch Q1, and the saturation inductance of thesaturable reactor SL1.

FIG. 6 is a structural view showing the transformer arranged in the DCconverter according to the first embodiment. The transformer shown inFIG. 6 has a core 20 having a rectangular external shape. The core 20has elongated gaps 22 a and 22 b to form magnetic paths 21 a, 21 b, and21 c, the gaps 22 a and 22 b being extended in parallel with the lengthof the magnetic paths. Around a core part 20 a of the core 20, theprimary winding 5 a and secondary winding 5 b are wound. To provide aleakage inductor, the primary winding 5 a and secondary winding 5 b aredividedly wound.

Two recesses 20 b are formed on an outer circumferential core, oppositeto a space between the primary winding 5 a and the secondary winding 5b. The recesses 20 b partly reduce the cross-sectional areas of themagnetic paths 21 b and 21 c in the outer circumferential core. Only thereduced parts magnetically saturate, to reduce a core loss when theprimary winding 5 a serves as the saturable reactor SL1.

Operation of the DC converter according to the first embodiment with theabove-mentioned configuration will be explained with reference to timingcharts of FIGS. 7, 8, and 10. The timing chart of FIG. 7 shows signalsat various parts of the DC converter according to the first embodiment.The timing chart of FIG. 8 shows the same signals of the DC converteraccording to the first embodiment when the switch Q1 is turned on. FIG.9 shows the B-H characteristic of the transformer in the DC converteraccording to the first embodiment. The timing chart of FIG. 10 shows acurrent to the saturable reactor in the DC converter according to thefirst embodiment.

FIGS. 7 and 8 show a terminal voltage Q1 v of the switch Q1, a currentQ1 i passed to the switch Q1, a terminal voltage Q2 v of the switch Q2,a current Q2 i passed to the switch Q2, a current Idc1 i passed to thepower supply source Idc1, and a current SL1 i passed to the saturablereactor SL1.

At time t1 (corresponding to t11 to t12), the switch Q1 is turned on.Then, a current passes in order of Vdc1, 5 a, Q1, and Vdc1. At the sametime, the secondary winding 5 b of the transformer T generates a voltageto pass a current in order of 5 b, D1, C4, and 5 b. When the switch Q1is turned on, the saturable reactor SL1 passes a current SL1 i toaccumulate energy in the inductor of the saturable reactor SL1.

The current SL1 i changes, as shown in FIG. 10, to a current value a(negative value) at time t1, to a current value b (negative value) attime t1 b, to a current value c (zero) at time t13, and to a currentvalue d (positive value) at time t2. On the B-H curve shown in FIG. 9,magnetic flux changes in order of Ba, Bb, Bc, and Bd. An operation rangeΔB of magnetic flux according to the present invention is as shown inFIG. 9, and the B-H curve has a saturation region Hs. Ba to Bg shown inFIG. 9 temporally correspond to a to g shown in FIG. 10.

At time t2, the switch Q1 is turned off. Then, the energy accumulated inthe saturable reactor SL1 charges the capacitor C1. At this time, theinductance of the saturable reactor SL1 and the capacitor C1 form avoltage resonance, to steeply increase the voltage of the capacitor C1,i.e., the voltage Q1 v of the switch Q1.

When the potential of the capacitor C1 becomes equal to the potential ofthe capacitor C3, the discharge of energy of the saturable reactor SL1makes the diode D4 conductive to pass a diode current that charges thecapacitor C3. At this time, the switch Q2 is turned on so that theswitch Q2 becomes a zero-voltage switch. From time t2 to time t20, thecurrent SL1 i changes from the current value d (positive value) to acurrent value e (zero). Accordingly, on the B-H curve, magnetic fluxchanges from Bd to Be.

Simultaneously with the energy discharge of the saturable reactor SL1,energy from the power supply source Idc1 is supplied to the capacitor C3to charge the capacitor C3. Namely, the capacitor C3 cumulativelyreceives both the energy from the power supply source Idc1 and theenergy from the saturable reactor SL1. Upon the completion of the energydischarge of the saturable reactor SL1 and the energy discharge of thepower supply source Idc1, the charging of the capacitor C3 stops.

From time t20 to time t3, the energy accumulated in the capacitor C3flows in order of C3, Q2, SL1, and C3 to reset the magnetic flux of thesaturable reactor SL1. The magnetic flux of the transformer T that isconnected in parallel with the saturable reactor SL1 similarly changes.

From time t20 to time t3, the energy accumulated in the capacitor C3 isreturned to the saturable reactor SL1, and therefore, the current SL1 ipassed through the saturable reactor SL1 becomes negative as shown inFIG. 10. Namely, the current SL1 i changes from the current value e(zero) to a current value f (negative value) in the period from t20 tot2 a. As a result, on the B-H curve shown in FIG. 9, magnetic fluxchanges from Be to Bf. An area S from time t2 to time t20 is equal to anarea S from time t20 to time t2 a. The area S corresponds to the energyof the saturable reactor SL1 accumulated in the capacitor C3.

From time t2 a to time t3, the current SL1 i changes from the currentvalue f (negative value) to a current value g (negative value). On theB-H curve shown in FIG. 9, magnetic flux changes from Bf to Bg in thesaturation region Hs. An area from time t2 a to time t3 corresponds tothe energy of the power supply source Idc1 accumulated in the capacitorC3.

Namely, the energy accumulated in the capacitor C3 is the sum of theenergy of the saturable reactor SL1 and the energy of the power supplysource Idc1, and therefore, the current SL1 i is increased by the energysupplied from the power supply source Idc1 at the time of resetting. Asa result, magnetic flux moves to the third quadrant to reach thesaturation region (Bf to Bg) to increase the current SL1 i, whichbecomes the maximum at time t3 (like at time t1). The current SL1 iincreases to a saturation current of the saturable reactor SL1 justbefore the end of the ON period of the switch Q2.

At time t3, the current Q2 i of the switch Q2 reaches the maximum. Atthis time, the switch Q2 is turned off to steeply discharge thecapacitor C1, which quickly becomes zero. At this time, the switch Q1 isturned on to achieve zero-voltage switching.

FIG. 11 is a circuit diagram showing the details of the DC converteraccording to the first embodiment. The first embodiment shown in FIG. 11forms the power supply source Idc1 from a series circuit (second seriescircuit) composed of a reactor (first reactor) L2 and a diode D6.

According to this embodiment, the reactor L2 accumulates energy when theswitch Q1 is turned on, and when the switch Q1 is turned off, the energyaccumulated in the reactor L2 is supplied to the capacitor C3 to chargethe capacitor C3. The power supply source Idc1 shown in FIG. 11 isappropriate for light load.

First Modification

FIG. 12 is a circuit diagram showing a first modification of the DCconverter according to the first embodiment. The first modificationshown in FIG. 12 forms the power supply source Idc1 from a reactor(second reactor) L3 that is connected in series with the primary winding5 a of the transformer T.

According to the first modification, the switch Q1 is turned on to passa current through the reactor L3 and accumulate energy in the reactorL3. When the switch Q1 is turned off, the energy is discharged in orderof L3, T, D4, C3, and L3. The energy is partly supplied through thesecondary winding 5 b of the transformer T to the load RL and is partlysupplied to the capacitor C3 to charge the capacitor C3. The powersupply source Idc1 shown in FIG. 12 is appropriate for heavy load.

Second Modification

FIG. 13 is a circuit diagram showing a second modification of the DCconverter according to the first embodiment. The second modificationshown in FIG. 13 combines the reactor L2 and diode D6 serving as thepower supply source Idc1 of FIG. 11 with the reactor L3 serving as thepower supply source Idc1 of FIG. 12, to cope with light load and heavyload.

The reactor L3 may be replaced with a leakage inductor of thetransformer T. The saturable reactor SL1 may be replaced with anexcitation inductance of the transformer T if the transformer employsthe core of FIG. 6 with a good saturation characteristic. This circuitcan control an output voltage with a fixed switching frequency toperform PWM control, thereby easily coping with a broadcastinginterference and the like.

As explained above, the embodiment can achieve zero-voltage switching,relax the rise and fall of voltage due to a resonant action, and realizea low-noise, high-efficiency DC converter.

In addition, the embodiment can improve the flux use ratio of atransformer core and reduce the size of the DC converter.

Second Embodiment

FIG. 14 is a circuit diagram showing a DC converter according to asecond embodiment. The DC converter according to the second embodimentemploys a secondary winding 5 b and a tertiary winding 5 c on thesecondary side of a transformer T, to provide two outputs. The secondaryside of the transformer T may have three or more windings to providethree or more outputs. Only the two-output configuration will beexplained.

The DC converter of this embodiment uses, in addition to the DCconverter of FIG. 12, the tertiary winding 5 c wound around the core ofthe transformer T, a diode D2, a capacitor C2, and a load RL2. Thetertiary winding 5 c and secondary winding 5 b are in-phase. A first endof the tertiary winding 5 c is connected to an anode of the diode D2,and a cathode of the diode D2 and a second end of the tertiary winding 5c are connected to the capacitor C2. The diode D2 and capacitor C2 forma rectifying/smoothing circuit. The capacitor C2 smoothes a rectifiedvoltage of the diode D2 and provides a DC output to the load RL2.

The primary winding 5 a and secondary winding 5 b are loosely coupled,and the primary winding 5 a and tertiary winding 5 c are looselycoupled. For example, the windings are separated from each other torealize the coarse coupling. The secondary winding 5 b and tertiarywinding 5 c are tightly coupled. For example, these windings are broughtcloser to each other to realize the dense coupling.

A control circuit 10 turns on and off a switch Q1 and a switch Q2alternately. When an output voltage of a load RL1 becomes equal to orlarger than a reference voltage, the ON-width of a pulse applied to theswitch Q1 is narrowed and the ON-width of a pulse applied to the switchQ2 is widened. Namely, when an output voltage of the load RL1 becomesequal to or greater than the reference voltage, the ON-width of a pulseto the switch Q1 is narrowed to maintain output voltages at respectiveconstant values.

A circuitry on the primary side of the transformer T is the same as thatof the first modification of the first embodiment. Namely, a powersupply source Idc1 is made of a reactor L3 that is connected in serieswith the primary winding 5 a of the transformer T.

In the DC converter according to the second embodiment, a voltage fromthe secondary winding 5 b is rectified and smoothed with the diode D1and capacitor C4, to supply DC power to the load RL1. A voltage from thetertiary winding 5 c is rectified and smoothed with the diode D2 andcapacitor C2, to supply DC power to the load RL2.

The primary winding 5 a and secondary winding 5 b are loosely coupled,and therefore, a leakage inductor on the primary side is large. Thesecondary winding 5 b and tertiary winding 5 c are tightly coupled, andtherefore, a leakage inductor on the secondary side is small. As aresult, a secondary output (the output of the secondary winding and theoutput of the tertiary winding) involves small variations with respectto light load and heavy load, to realize a good load variationcharacteristic. This improves a cross regulation on the secondary side.Since a satisfactory cross regulation is achieved among the plurality ofoutputs, auxiliary regulators can be omitted to realize a simplecircuitry.

To provide a plurality of outputs on the secondary side, another DCconverter (not shown) may be embodied based on the DC converter of FIG.14 added with a secondary circuitry (a tertiary winding 5 c, diodes D3and D4, a reactor L2, and a capacitor C2) that is the same as thesecondary circuitry (the secondary winding 5 b, diodes D1 and D2,reactor L1, and capacitor C4) of the DC converter shown in FIG. 1.

Since the reactors L1 and L2 are large, they may be wound around thesame core. This, however, worsens a cross regulation on the secondaryside. Matching a turn ratio of the secondary winding 5 b and reactor L1with a turn ratio of the tertiary winding 5 c and reactor L2 isdifficult because the numbers of turns are small.

The second embodiment shown in FIG. 14 does not employ the reactors L1and L2. A leakage inductor on the secondary side is small, and a leakageinductance between the primary side and the secondary side is large.This improves a cross regulation on the secondary side and realizes asimple circuitry.

Third Embodiment

A DC converter according to a third embodiment will be explained. The DCconverters of the first and second embodiments employ normally-OFF-typeMOSFETs as switches. The normally-OFF-type switch is a switch that is inan OFF state when a power source is OFF. On the other hand, anormally-ON-type switch such as a SIT (static induction transistor) is aswitch that is in an ON state when a power source is OFF. Thenormally-ON-type switch shows a high switching speed and lowON-resistance, and therefore, is an ideal element for use with a powerconverter such as a switching power source because it is expected toreduce a switching loss and improve efficiency.

The normally-ON-type switching element is in an ON state when a powersource is turned on, and therefore, the switch will be short-circuitedto become inactive. Due to this, the normally-ON-type switch has limitedusage.

The DC converter according to the third embodiment employs theconfiguration of the DC converter of the first embodiment and employs anormally-ON-type switch as the switch Q1. When an AC power source isturned on, a voltage due to a voltage drop of a rush current limitingresistor, which is inserted to reduce a rush current of an inputsmoothing capacitor, is used as a reverse bias voltage for thenormally-ON-types witch. This configuration solves the problem thatoccurs when the power source is turned on.

FIG. 15 is a circuit diagram showing the DC converter according to thethird embodiment. The DC converter shown in FIG. 15 has theconfiguration of the DC converter according to the first embodimentshown in FIG. 11. In addition, the DC converter of the third embodimentrectifies an AC voltage from an AC power source Vac1 with a full-waverectifying circuit (input rectifying circuit) B1, converts the obtainedvoltage into an other DC voltage, and outputs the DC voltage. Between anoutput terminal P1 and another output terminal P2 of the full-waverectifying circuit B1, there is connected a series circuit composed ofan input smoothing capacitor C5 and a rush current limiting resistor R1.The AC power source Vac1 and full-wave rectifying circuit B1 correspondto the DC power source Vdc1 shown in FIG. 11.

The output terminal P1 of the full-wave rectifying circuit B1 isconnected through a primary winding 5 a of a transformer T to thenormally-ON-type switch Q1 n such as a SIT. The switch Q1 n is turned onand off under PWM control by a control circuit 11. Switches such as Q2other than the switch Q1 n are normally-OFF-type switches.

Both ends of the rush current limiting resistor R1 are connected to aswitch S1. The switch S1 is a semiconductor switch such as anormally-OFF-type MOSFET or BJT (bipolar junction transistor) and isturned on according to a short-circuit signal from the control circuit11.

Both ends of the rush current limiting resistor R1 are also connected toa starting power source 12 composed of a capacitor C6, a resistor R2,and a diode D5. The starting power source 12 takes a voltage generatedat both ends of the rush current controlling resistor R1 and uses aterminal voltage of the capacitor C6 as a reverse bias voltage to beapplied through a terminal “a” of the control circuit 11 to a gate ofthe switch Q1 n. A voltage charged in the input smoothing capacitor C5is supplied to the control circuit 11.

When the AC power source Vac1 is turned on, the control circuit 11starts in response to a voltage supplied from the capacitor C6 andsupplies, as a control signal, a reverse bias voltage from a terminal“b” to the gate of the switch Q1 n, to turn off the switch Q1 n. Thecontrol signal is a pulse signal between, for example, −15 V and 0 V.The voltage of −15V turns off the switch Q1 n, and the voltage of 0 Vturns on the switch Q1 n.

After the completion of charging the input smoothing capacitor C5, thecontrol circuit 11 provides a control signal composed of pulse signalsof 0 V and −15 V from the terminal b to the gate of the switch Q1 n tomake the switch Q1 n conduct a switching operation. A predetermined timeafter making the switch Q1 n perform the switching operation, thecontrol circuit 11 outputs a short-circuit signal to the gate of theswitch S1, to turn on the switch S1.

A first end of an auxiliary winding 5 d (the number of turns being n4)of the transformer T is connected to a first end of the switch Q1 n, afirst end of a capacitor C7, and the control circuit 11. A second end ofthe auxiliary winding 5 d is connected to a cathode of a diode D7. Ananode of the diode D7 is connected to a second end of the capacitor C7and a terminal c of the control circuit 11. The auxiliary winding 5 d,diode D7, and capacitor C7 form a normal operation power source 13. Thenormal operation power source 13 supplies a voltage generated by theauxiliary winding 5 d to the control circuit 11 via the diode D7 andcapacitor C7.

Operation of the DC converter according to the third embodiment with theabove-mentioned configuration will be explained with reference to FIGS.15 to 17.

In FIG. 17, Vac1 is an AC voltage of the AC power source Vac1, an inputcurrent is a current passed through the AC power source Vac1, R1V is avoltage generated by the rush current limiting resistor R1, C5V is avoltage of the input smoothing capacitor C5, C6V is a voltage of thecapacitor C6, an output voltage is a voltage of the capacitor C4, and acontrol signal is a signal outputted from the terminal b of the controlcircuit 11 to the gate of the switch Q1 n.

At time t0, the AC power source Vac1 is turned on. An AC voltage fromthe AC power source Vac1 is full-wave-rectified with the full-waverectifying circuit B1. At this time, the normally-ON-type switch Q1 n isin an ON state, and the switch S1 is in an OFF state. As a result, avoltage from the full-wave rectifying circuit B1 is fully appliedthrough the input smoothing capacitor C5 to the rush current limitingresistor R1 ((1) in FIG. 16).

The voltage generated by the rush current limiting resistor R1 isaccumulated through the diode D5 and resistor R2 in the capacitor C6((2) in FIG. 16). At this time, the potential of a terminal f of thecapacitor C6 becomes, for example, zero, and a terminal g of thecapacitor C6 becomes, for example, negative potential. As a result, thecapacitor C6 provides a negative voltage (reverse bias voltage) as shownin FIG. 17. The negative voltage of the capacitor C6 is supplied to theterminal a of the control circuit 11.

When the voltage of the capacitor C6 reaches a threshold voltage THL ofthe switch Q1 n (at time t1 in FIG. 17), the control circuit 11 outputsa control signal of −15 V from the terminal b to the gate of the switchQ1 n ((3) in FIG. 16). This changes the switch Q1 n to an OFF state.

Due to the voltage from the full-wave rectifying circuit B1, the inputsmoothing capacitor C5 is charged ((4) in FIG. 16), and therefore, thevoltage of the input smoothing capacitor C5 increases to complete thecharging of the input smoothing capacitor C5.

At time t2, the control circuit 11 starts a switching operation. Atfirst, the terminal b outputs a control signal of 0 V to the gate of theswitch Q1 n ((5) in FIG. 16). This puts the switch Q1 n in an ON state.The output terminal P1 of the full-wave rectifying circuit B1 passes acurrent through the primary winding 5 a of the transformer T to theswitch Q1 n ((6) in FIG. 16), to accumulate energy in the primarywinding 5 a of the transformer T. At this time, a secondary winding 5 bgenerates a voltage to pass a current in order of 5 b, D1, C4, and 5 b,to supply power to a load RL.

The auxiliary winding 5 d, which is electromagnetically coupled with theprimary winding 5 a of the transformer T, also generates a voltage thatis supplied through the diode D7 and capacitor C7 to the control circuit11 ((7) in FIG. 16). Due to this, the control circuit 11 cancontinuously operate to continue the switching operation of the switchQ1 n.

At time t3, the terminal b outputs a control signal of −15 V to the gateof the switch Q1 n, to turn off the switch Q1 n. Also at time t3, theinductance of a saturable reactor SL1 and a resonant capacitor C1resonate to increase the voltage of the switch Q1 n and decrease thevoltage of the switch Q2.

At time t3, the control circuit 11 outputs a short-circuit signal to theswitch S1 to turn on the switch S1 and short-circuit both ends of therush current limiting resistor R1 ((8) in FIG. 16). This reduces a lossof the rush current limiting resistor R1.

Time t3 is based on an elapsed time from the time when the AC powersource Vac1 is turned on (time t0) and is, for example, about five timesor more greater than a time constant (τ=C5·R1) of the input smoothingcapacitor C5 and rush current controlling resistor R1. Thereafter, theswitch Q1 n repeats the ON/OFF switching operation. After the switch Q1n starts the switching operation, the switches Q1 n and Q2 operate likethe switches Q1 and Q2 of the DC converter according to the firstembodiment shown in FIG. 11, i.e., like the operation shown in thetiming charts of FIGS. 7 and 8.

In this way, the DC converter according to the third embodiment providesthe effect of the first embodiment. In addition, the control circuit 11turns off the switch Q1 n with a voltage generated by the rush currentlimiting resistor R1 when the AC power source Vac1 is turned on. Afterthe input smoothing capacitor C5 is charged, the control circuit 11starts the ON/OFF switching operation of the switch Q1 n. Thiseliminates the problem that a normally-OFF-type switch is not properlystarted when a power source is turned on. Namely, this embodiment canemploy a normally-ON-type semiconductor switch to provide a DC converterof low loss and high efficiency.

Although the third embodiment adds a normally-ON circuit to theapparatus of the first embodiment, the normally-ON circuit may be addedto the apparatus of the second embodiment.

Fourth Embodiment

A DC converter according to a fourth embodiment will be explained.According to the DC converters of the first and second embodiments, thecurrent Q1 i of the switch Q1 steeply inclines as shown in a timingchart of FIG. 18 when an input voltage is high (ii), i.e., when an inputvoltage widely varies. This results in increasing a peak current anddrastically shortening an ON-width. To solve this problem, theinductance of the reactor L3 (for example a leakage inductor between theprimary and secondary windings) on the primary side may be increased.

However, energy accumulated in the reactor L3 when the switch Q1 is ONis accumulated in the snubber capacitor C3 when the switch Q1 is OFF.When the switch Q1 is turned on next time, the accumulated energy isreturned to the input. As a result, the energy accumulated in thereactor L3 increases to decrease efficiency. If the input voltage variesin a wide range, the peak current of the switch Q1 increases on the highinput voltage side to increase energy returning to the input and greatlydeteriorate efficiency.

The DC converter according to the fourth embodiment increases theinductance of a reactor connected in series with a primary winding of atransformer and employs an auxiliary transformer that is used to add asecond return circuit to return energy accumulated in the reactor duringthe ON-time of the switch Q1 to the secondary side.

FIG. 19 is a circuit diagram showing the DC converter according to thefourth embodiment. The DC converter according to the fourth embodimentshown in FIG. 19 differs from the DC converter according to the firstembodiment shown in FIG. 5 in the transformer T and in peripheralcircuits around the transformer T. The different parts will mainly beexplained.

A first end of a primary winding 5 a of the transformer is connected toa first end of a reactor (third reactor) L4. A second end of the reactorL4 is connected to a first end of a switch Q1. A second end (marked witha dot) of the primary winding 5 a of the transformer T is connected to afirst end (marked with a dot) of a primary winding 5 a 2 (the number ofturns being n1) of an auxiliary transformer Tb. A second end of theprimary winding 5 a 2 of the auxiliary transformer Tb is connected tothe second end of the reactor L4. Accordingly, a saturable reactor SL1is connected in parallel through the reactor L4 to the primary winding 5a. The primary winding 5 a and switch Q1 form a series circuit throughthe reactor L4, the series circuit being connected in parallel with a DCpower source Vdc1.

A second end (marked with a dot) of a secondary winding 5 b of thetransformer T is connected to a first end (marked with a dot) of asecondary winding 5 b 2 (the number of turns being n2) of the auxiliarytransformer Tb. A second end of the secondary winding 5 b 2 of theauxiliary transformer Tb is connected to an anode of a diode D42. Acathode of the diode D42 is connected to a cathode of a diode D1 and afirst end of a capacitor C4. A second end of the capacitor C4 isconnected to the first end of the secondary winding 5 b of thetransformer T. The auxiliary transformer Tb returns energy accumulatedin the reactor L4 when the switch Q1 is ON to the secondary side whenthe switch Q1 is OFF.

Operation of the DC converter according to the fourth embodiment withthe above-mentioned configuration will be explained with reference toFIG. 19 and timing charts of FIGS. 20 and 21. The timing chart of FIG.20 shows signals at various parts of the DC converter according to thefourth embodiment. The timing chart of FIG. 21 shows signals at thevarious parts of the DC converter according to the fourth embodimentwhen the switch Q1 is turned on.

FIGS. 20 and 21 show a terminal voltage Q1 v of the switch Q1, a currentQ1 i passing through the switch Q1, a terminal voltage Q2 v of a switchQ2, a current Q2 i passing through the switch Q2, and a current SL1 ipassing through the saturable reactor SL1.

At time t1, the switch Q1 is turned on. Like the first embodiment, acurrent passes in order of Vdc1, 5 a, L4, Q1, and Vdc1. At the sametime, the secondary winding 5 b of the transformer T generates a voltageto pass a current in order of 5 b, D1, C4, and 5 b like the firstembodiment. As shown in FIG. 20, a current of the diode D1 linearlyincreases.

At time t2, the switch Q1 is turned off. Like the first embodiment,energy accumulated in the saturable reactor SL1 serving as an energykeeping element and energy in a power supply source Idc1 charge acapacitor C3. In addition, energy accumulated in the reactor L4 isreturned through the auxiliary transformer Tb to the secondary side.Namely, a current is passed in order of L4, 5 a 2, 5 a, and L4 to inducea voltage on the secondary winding 5 b 2 of the auxiliary transformerTb. Then, a current is passed in order of 5 b 2, D42, C4, 5 b, and 5 b2. As shown in FIG. 20, a current is passed through the diode D42 fromtime t2 to time t3.

When the switch Q1 is OFF, the primary winding 5 a of the transformer Thas a voltage V11, the primary winding 5 a 2 of the auxiliarytransformer Tb has a voltage V21, and the reactor L4 has a voltage V12.Then, an expression (1) is established.V11+V12=V21  (1)

If the transformer T and auxiliary transformer Tb have a turn ratio a,the expression (1) is written as follows:aV21=aV11=aV12  (2)

Namely, the voltage aV12, i.e., a voltage multiplied by the turn ratioof the reactor L4 is rectified by the diode D42 and is supplied to thecapacitor C4.

In this way, the reactor L4 connected in series with the primary winding5 a of the transformer T is provided with an increased inductance toreturn energy accumulated when the switch Q1 is ON to the secondary sidethrough the auxiliary transformer Tb, to improve efficiency. Due to thediodes D1 and D42, a secondary current continuously flows during ON andOFF periods, to reduce a ripple current of the smoothing capacitor C4.

The saturable reactor SL1 is connected in parallel with the primarywinding 5 a of the transformer T, and there is provided the power supplysource Idc1, to achieve a zero-voltage switching operation. Thezero-voltage switching operation is the same as that of the DC converterof the first embodiment, and therefore, is not explained here.

First Modification

FIG. 22 is a circuit diagram showing a DC converter according to a firstmodification of the fourth embodiment. The first modification shown inFIG. 22 provides the transformer T with a primary winding 5 a (thenumber of turns being n1), a secondary winding 5 b (the number of turnsbeing n2), and a tertiary winding 5 c (having the number of turns of n3and corresponding to the secondary winding 5 b 2 of the auxiliarytransformer Tb). The primary winding 5 a and secondary winding 5 b arein-phase, and the primary winding 5 a and tertiary winding 5 c haveopposite phases.

According to this embodiment, the secondary winding 5 b of thetransformer T is loosely coupled with the primary winding 5 a, and aleakage inductance between the primary winding 5 a and the secondarywinding 5 b serves as a reactor L4 connected in series with thetransformer T. Namely, the leakage inductance between the primarywinding 5 a and the secondary winding 5 b serves as the reactor L4 ofFIG. 19, to return energy to the secondary side. This means that theauxiliary transformer Tb shown in FIG. 19 can be coupled with thetransformer T shown in FIG. 22. An example of coupling the auxiliarytransformer Tb with the transformer T will be explained later withreference to FIG. 25.

The power supply source Idc1 is formed from a series circuit (secondseries circuit) composed of a reactor L2 and a diode D6. The operationand effect of the power supply source Idc1 are the same as those of theDC converter according to the first embodiment shown in FIG. 11, andtherefore, will not be explained again.

Second Modification

FIG. 23 is a circuit diagram showing a second modification of the DCconverter according to the fourth embodiment. The second modificationshown in FIG. 23 differs from the embodiment of FIG. 22 only in thestructure of a power supply source Idc1. Namely, the power supply sourceIdc1 of the second modification is formed from a reactor L3 connected inseries with the primary winding 5 a of the transformer T. The operationand effect of the power supply source Idc1 are the same as those of thefirst modification of the DC converter according to the firstembodiment, and therefore, are not explained here.

The reactor L2 and diode D6 of the power supply source Idc1 shown inFIG. 22 may be combined with the reactor L3 serving as the power supplysource Idc1 shown in FIG. 23. This can cope with light load as well asheavy load.

The reactor L3 may be replaced with a leakage inductor of thetransformer T. The saturable reactor SL1 may be replaced with anexcitation inductance of a transformer T shown in FIG. 25 employing acore of good saturation characteristic. This circuit can control anoutput voltage with a fixed switching frequency and PWM control, toeasily cope with a broadcasting interference.

Fifth Embodiment

A DC converter according to a fifth embodiment will be explained. FIG.24 is a circuit diagram showing the DC converter according to the fifthembodiment. The DC converter according to the fifth embodiment employs atransformer having a secondary winding 5 b and a quaternary winding 5 don the secondary side to provide two outputs. The secondary side of thetransformer T may have three or more windings to provide three or moreoutputs. Here, only two outputs will be explained.

The DC converter according to this embodiment includes, in addition tothe components of the DC converter shown in FIG. 23, the quaternarywinding 5 d wound around a core of the transformer T, a diode D55, acapacitor C2, and a load RL2. The quaternary winding 5 d is in-phasewith respect to the secondary winding 5 b. A first end of the quaternarywinding 5 d is connected to an anode of the diode D55. A cathode of thediode D55 and a second end of the quaternary winding 5 d are connectedto the capacitor C2. The diode D55 and capacitor C2 form arectifying/smoothing circuit. The capacitor C2 smoothes a rectifiedvoltage of the diode D55 and provides a DC output to the load RL2.

A primary winding 5 a and the quaternary winding 5 d are looselycoupled. For example, these windings are spaced apart from each other torealize the coarse coupling. The secondary winding 5 b and quaternarywinding 5 d are tightly coupled. For example, these windings are broughtcloser to each other to realize the dense coupling.

A control circuit 10 turns on and off switches Q1 and Q2 alternately.When an output voltage of a load RL1 becomes equal to or greater than areference voltage, the ON-width of a pulse applied to the switch Q1 isnarrowed and the ON-width of a pulse applied to the switch Q2 iswidened. Namely, when an output voltage of the load RL1 becomes equal toor greater than the reference voltage, the ON-width of a pulse appliedto the switch Q1 is narrowed to control each output voltage to aconstant value.

In this way, according to the DC converter of the fifth embodiment, avoltage from the secondary winding 5 b is rectified and smoothed with adiode D1 and a capacitor C4 and DC power is supplied to the load RL1.Also, a voltage from the quaternary winding 5 d is rectified andsmoothed with the diode D55 and capacitor C2 and DC power is supplied tothe load RL2.

The primary winding 5 a and secondary winding 5 b are loosely coupled toincrease a leakage inductance on the primary side. The secondary winding5 b and quaternary winding 5 d are tightly coupled, to reduce a leakageinductance on the secondary side. As a result, outputs of the secondaryside (output of the secondary winding and output of the quaternarywinding) show little variation under light load as well as under heavyload, to achieve a good load variation characteristic. This improves across regulation on the secondary side. A cross regulation among theplurality of outputs is satisfactory, and therefore, an auxiliaryregulator can be omitted to simplify a circuitry.

FIG. 25 is a structural view showing a transformer for the DC converteraccording to any one of the fourth and fifth embodiments. Thetransformer shown in FIG. 25 has a core 30 with substantially arectangular external shape. The core 30 has magnetic paths 31 a, 31 b,and 31 c formed by elongate gaps 32 a and 32 b extended along the lengthof the magnetic paths. The core 30 has a core part 30 a around which aprimary winding 5 a and a tertiary winding 5 c are wound close to eachother. This forms a slight leakage inductance between the primarywinding and the tertiary winding, so that the leakage inductance is usedas the reactor L3. The core 30 defining the magnetic path 31 b forms apath core 30 c and a gap 31. An outer circumferential core defining themagnetic path 31 c is wound with a secondary winding 5 b. A quaternarywinding 5 d is wound close to the secondary winding 5 b. Namely, thepath core 30 c functions to loosely couple the primary winding 5 a andsecondary winding 5 b (also the quaternary winding 5 d), to increase aleakage inductance. This large leakage inductance is used as the reactorL4.

Two recesses 30 b are formed on the outer circumferential core betweenthe primary winding 5 a and the secondary winding 5 b. The recesses 30 bpartly reduce the cross-sectional area of the outer circumferentialcore, so that only the narrowed parts may saturate to reduce a coreloss. This saturation primary winding 5 a may be used as the saturablereactor SL1.

In this way, the shape of the core of the transformer T and the windingsthereof may be configured to combine, on the single core 30, thetransformer T with the auxiliary transformer Tb for returning energy ofthe reactor L4 to the secondary side. In addition, the path core 30 c isformed. This results in providing a large leakage inductance. Since thetransformer and reactor are combined together, the DC converter is smalland is manufacturable at low cost.

Sixth Embodiment

A DC converter according to a sixth embodiment will be explained. FIG.26 is a circuit diagram showing the DC converter according to the sixthembodiment. The DC converter according to the sixth embodiment employs asynchronous rectifier in a secondary circuitry of a transformer. Thetransformer outputs a rectangular waveform, and the embodiment increasesa conduction ratio for synchronous rectification, to reduce a rectifierloss for a low output voltage and improve rectification efficiency.

The DC converter according to the embodiment shown in FIG. 26 is thesame as the DC converter according to the second modification of thefourth embodiment shown in FIG. 23 except the structure of the secondarycircuitry of the transformer T. Accordingly, the same parts arerepresented with the same reference marks, and only the structure of thesecondary circuitry of the transformer T will be explained. A primarywinding 5 a and a secondary winding 5 b are loosely coupled, and thesecondary winding 5 b and a tertiary winding 5 c are tightly coupled.

A first end (marked with a dot) of the secondary winding 5 b of thetransformer T is connected to a first end of a capacitor C4, and asecond end of the secondary winding 5 b of the transformer T isconnected through a switch (third switch) Q3 made of a FET to a secondend of the capacitor C4. A first end (marked with a dot) of the tertiarywinding 5 c of the transformer T is connected through a switch (fourthswitch) Q4 made of a FET to the second end of the capacitor C4. A secondend of the tertiary winding 5 c of the transformer T is connected to thesecond end of the secondary winding 5 b of the transformer T.

The first end of the tertiary winding 5 c of the transformer T isconnected to a gate of the switch Q3, and the second end of the tertiarywinding 5 c of the transformer T is connected to a gate of the switchQ4. The switch Q3 is connected in parallel with a diode D61, and theswitch Q4 is connected in parallel with a diode D62. These elements formthe synchronous rectifying circuit. The capacitor C4 forms a smoothingcircuit. The rectifying/smoothing circuit rectifies and smoothes avoltage (ON/OFF-controlled pulse voltage) induced on the secondarywinding 5 b and tertiary winding 5 c of the transformer T and outputs DCoutput to a load RL.

Operation of the DC converter according to the sixth embodiment with theabove-mentioned configuration will be explained with reference to atiming chart shown in FIG. 27. In FIG. 27, Q1 v is a terminal(drain-source) voltage of a switch Q1, Q1 i is a current (drain current)passing through the switch Q1, Q2 v is a terminal voltage of a switchQ2, Q2 i is a current passing through the switch Q2, Q3 i is a currentpassing through the switch Q3, Q4 i is a current passing through theswitch Q4, SL1 i is a current passing through a saturable reactor SL1,and VT is a terminal voltage of the secondary winding 5 b of thetransformer T.

In period T1 (or period T8, corresponding to time t0 to t1 in FIG. 20and time t2 to time t3 in FIG. 20), the switch Q1 is OFF and the switchQ2 is ON. As a result, the switch Q2 passes a current and the switch Q1passes no current. At this time, energy accumulated in a leakageinductor between the primary and secondary windings of the transformer Tgenerates voltage on the tertiary winding 5 c (negative on the sidemarked with a dot of the tertiary winding 5 c and positive on the otherside). As a result, the positive voltage is applied to the gate of theswitch Q4 to turn on the switch Q4, and the negative voltage is appliedto the gate of the switch Q3 to turn off the switch Q3. A current ispassed in order of 5 c, 5 b, C4, Q4, and 5 c, and an output voltage isgenerated on the load RL.

In a period from T2 to T4 (corresponding to time t1 in FIG. 20), theswitch Q2 is changed from the ON state to an OFF state, and the switchQ1 is changed from the OFF state to an ON state. Accordingly, thesaturation inductance of the saturable reactor SL1, the inductance of areactor L3, and a capacitor C1 resonate. The resonance decreases thevoltage of the switch Q1 and increases the voltage of the switch Q2(period T2). When the switch Q1 decreases close to a zero voltage(period T3), the switch Q1 is turned on, and therefore, a current ispassed through the switch Q1 (period T4).

In period T5 (corresponding to time t1 to time t2 of FIG. 20), theswitch Q1 is ON and the switch Q2 is OFF. At this time, a DC powersource Vdc1 supplies a current through the primary winding 5 a of thetransformer T to the switch Q1 to accumulate energy in the primarywinding 5 a (positive on the side marked with a dot of the primarywinding 5 a and negative on the other side). This energy generatesvoltage on the secondary winding 5 b and tertiary winding 5 c (positiveon each side marked with a dot of the secondary winding 5 b and tertiarywinding 5 c and negative on the other sides). Accordingly, the positivevoltage is applied to the gate of the switch Q3 to turn on the switchQ3, and the negative voltage is applied to the gate of the switch Q4 toturn off the switch Q4. Then, a current passes in order of 5 b, C4, Q3,and 5 b to supply DC power to the load RL. When the switch Q1 is turnedon, the saturable reactor SL1 passes the current SL1 i to accumulateenergy in the inductor of the saturable reactor SL1.

In period T6 (corresponding to time t2 of FIG. 20), the switch Q1 ischanged from the ON state to an OFF state. In period T6, the inductanceof the saturable reactor SL1, the inductance of the reactor L3, and theresonant capacitor C1 resonate. The resonance steeply increases thevoltage of the switch Q1.

In period T7 (corresponding to time t2 of FIG. 20), a diode D4 turns onafter the switch Q1 is turned off, and the diode D4 passes a current. Asa result, the energy in the saturable reactor SL1 and the energy in thereactor L3 are accumulated through the diode D4 in the snubber capacitorC3. During the ON period of the diode D4, the switch Q2 is turned on sothat the switch Q2 functions as a zero-voltage switch.

In this way, the DC converter according to the sixth embodiment providesthe effect of the fourth embodiment. In addition, the sixth embodimentemploys the synchronous rectifier in the secondary circuitry of thetransformer T. The transformer outputs a rectangular waveform, which isapplied to the gate of the synchronous rectifying element to make itconductive substantially during the whole period. Then, no currentpasses through the parallel-connected diode, and therefore, therectification is carried out without loss. This is effective for a lowoutput voltage such as 5 V or 3.3 V.

Seventh Embodiment

A DC converter according to a seventh embodiment will be explained. TheDC converter according to the seventh embodiment has the structure ofthe DC converter according to the fourth embodiment, and like the thirdembodiment, employs a normally-ON-type switch as a switch Q1. When an ACpower source is turned on, a voltage due to a voltage drop of a rushcurrent limiting resistor, which is inserted to reduce a rush current ofan input smoothing capacitor, is used as a reverse bias voltage for thenormally-ON-type switch, thereby solving the problem that occurs whenthe power source is turned on.

FIG. 28 is a circuit diagram showing the DC converter according to theseventh embodiment. The DC converter shown in FIG. 28 has the structureof the first modification of the DC converter according to the fourthembodiment shown in FIG. 22. Like the third embodiment shown in FIG. 15,an AC power source Vac1 supplies an AC voltage, which is rectified witha full-wave rectifying circuit (input rectifying circuit) B1 and isconverted into a DC output voltage. Between output ends P1 and P2 of thefull-wave rectifying circuit B1, there is connected a series circuitcomposed of an input smoothing capacitor C5 and a rush current limitingresistor R1. The AC power source Vac1 and full-wave rectifying circuitB1 correspond to the DC power source Vdc1 shown in FIG. 22.

The output end P1 of the full-wave rectifying circuit B1 is connectedthrough a primary winding 5 a of a transformer T to a normally-ON-typeswitch Q1 n such as a SIT. The switch Q1 n is turned on and off underPWM control by a control circuit 11. Switches such as Q2 other than theswitch Q1 n are normally-OFF-type switches.

Both ends of the rush current limiting resistor R1 are connected to aswitch S1. The switch S1 is a semiconductor switch such as anormally-OFF-type MOSFET or BJT (bipolar junction transistor) and isturned on in response to a short-circuit signal from the control circuit11.

Both ends of the rush current limiting resistor R1 is also connected toa starting power source 12 consisting of a capacitor C6, a resistor R2,and a diode D5. The starting power source 12 picks up a terminal voltageof the rush current limiting resistor R1 and uses a terminal voltage ofthe capacitor C6 as a reverse bias voltage to be applied through aterminal “a” of the control circuit 11 to a gate of the switch Q1 n. Avoltage charged in the input smoothing capacitor C5 is supplied to thecontrol circuit 11.

When the AC power source Vac1 is turned on, the control circuit 11 isactivated according to a voltage supplied from the capacitor C6 andsupplies, as a control signal, a reverse bias voltage from a terminal“b” to the gate of the switch Q1 n to turn off the switch Q1 n. Thecontrol signal is a pulse signal of, for example, −15 V or 0 V in whichthe voltage of −15 V turns off the switch Q1 n and the voltage of 0 Vturns on the switch Q1 n.

After the completion of charging the input smoothing capacitor C5, thecontrol circuit 11 provides a control signal composed of pulse signalsof 0 V and −15 V from the terminal b to the gate of the switch Q1 n tomake the switch Q1 n perform a switching operation. A predetermined timeafter making the switch Q1 n perform the switching operation, thecontrol circuit 11 provides a short-circuit signal to the gate of theswitch S1 to turn on the switch S1.

A first end of an auxiliary winding 5 d (the number of turns being n4)of the transformer T is connected to a first end of the switch Q1 n, afirst end of a capacitor C7, and the control circuit 11. A second end ofthe auxiliary winding 5 d is connected to a cathode of a diode D7. Ananode of the diode D7 is connected to a second end of the capacitor C7and a terminal c of the control circuit 11. The auxiliary winding 5 d,diode D7, and capacitor C7 form a normal operation power source 13. Thenormal operation power source 13 supplies a voltage generated by theauxiliary winding 5 d to the control circuit 11 through the diode D7 andcapacitor C7.

Operation of the DC converter according to the seventh embodiment withthe above-mentioned configuration will be explained with reference toFIGS. 28 and 29.

In FIG. 29, Vac1 is an AC voltage of the AC power source Vac1, an inputcurrent is a current passed through the AC power source Vac1, R1V is avoltage generated by the rush current limiting resistor R1, C5V is avoltage of the input smoothing capacitor C5, C6V is a voltage of thecapacitor C6, an output voltage is a voltage of a capacitor C4, and acontrol signal is a signal supplied from the terminal b of the controlcircuit 11 to the gate of the switch Q1 n.

At time t0, the AC power source Vac1 is turned on. The AC voltage of theAC power source Vac1 is full-wave-rectified with the full-waverectifying circuit B1. At this time, the normally-ON-type switch Q1 n isin an ON state, and the switch S1 is in an OFF state. As a result, thevoltage from the full-wave rectifying circuit B1 is fully appliedthrough the input smoothing capacitor C5 to the rush current limitingresistor R1 ((1) in FIG. 28).

The voltage generated by the rush current limiting resistor R1 isaccumulated through the diode D5 and resistor R2 in the capacitor C6((2) in FIG. 28). At this time, the potential of a terminal f of thecapacitor C6 becomes, for example, zero, and a terminal g of thecapacitor C6 becomes, for example, negative potential. As a result, thevoltage of the capacitor C6 becomes negative (reverse bias voltage) asshown in FIG. 29. The negative voltage of the capacitor C6 is suppliedto the terminal a of the control circuit 11.

When the voltage of the capacitor C6 reaches a threshold voltage THL ofthe switch Q1 n (time t1 in FIG. 29), the control circuit 11 outputs acontrol signal of −15 V from the terminal b to the gate of the switch Q1n ((3) in FIG. 28). As a result, the switch Q1 n shifts to an OFF state.

Due to the voltage from the full-wave rectifying circuit B1, the inputsmoothing capacitor C5 is charged ((4) in FIG. 28), and therefore, thevoltage of the input smoothing capacitor C5 increases to complete thecharging of the input smoothing capacitor C5.

At time t2, the control circuit 11 starts a switching operation. Atfirst, the terminal b outputs a control signal of 0 V to the gate of theswitch Q1 n ((5) in FIG. 28). This puts the switch Q1 n in an ON state.The output terminal P1 of the full-wave rectifying circuit B1 passes acurrent through the primary winding 5 a of the transformer T to theswitch Q1 n ((6) in FIG. 28), to accumulate energy in the primarywinding 5 a of the transformer T. At this time, a secondary winding 5 bgenerates a voltage to pass a current in order of 5 b, D1, C4, and 5 b.As a result, power is supplied to a load RL.

The auxiliary winding 5 d, which is electromagnetically coupled with theprimary winding 5 a of the transformer T, also generates a voltage thatis supplied through the diode D7 and capacitor C7 to the control circuit11 ((7) in FIG. 28). Due to this, the control circuit 11 cancontinuously operate to continue the switching operation of the switchQ1 n.

At time t3, the terminal b outputs a control signal of −15 V to the gateof the switch Q1 n. As a result, the switch Q1 n turns off at time t3,and energy accumulated in a leakage inductor between the primary andsecondary windings passes a current through 5 c, D42, C4, 5 b, and 5 cin this order. Also at time t3, the inductance of a saturable reactorSL1 and a resonant capacitor C1 resonate to increase the voltage of theswitch Q1 n and decrease the voltage of the switch Q2.

At time t3, the control circuit 11 outputs a short-circuit signal to theswitch S1 to turn on the switch S1 and short-circuit the ends of therush current limiting resistor R1 ((8) in FIG. 28). This reduces a lossby the rush current limiting resistor R1.

Time t3 is based on an elapsed time from the time when the AC powersource Vac1 is turned on (time t0) and is, for example, about five timesor more greater than a time constant (τ=C5·R1) of the input smoothingcapacitor C5 and rush current controlling resistor R1. Thereafter, theswitch Q1 n repeats the ON/OFF switching operation. After the switch Q1n starts the switching operation, the switches Q1 n and Q2 operate likethe switches Q1 and Q2 of the DC converter according to the fourthembodiment shown in FIG. 22, i.e., like the operation shown in thetiming charts of FIGS. 20 and 21.

In this way, the DC converter according to the seventh embodimentprovides the effect of the fourth embodiment. In addition, like thethird embodiment, the control circuit 11 turns off the switch Q1 n witha voltage generated by the rush current limiting resistor R1 when the ACpower source Vac1 is turned on. After the input smoothing capacitor C5is charged, the control circuit 11 starts the ON/OFF switching operationof the switch Q1 n. This solves the problem that occurs when the powersource is turned on. Namely, this embodiment can employ anormally-ON-type semiconductor switch to provide a DC converter of lowloss and high efficiency.

Although the seventh embodiment adds a normally-ON circuit to theapparatus of the fourth embodiment, the normally-ON circuit may be addedto the apparatus of the fifth embodiment or to the apparatus of thesixth embodiment.

As explained above, this embodiment can achieve zero-voltage switching,relax the rise and fall of voltage with a resonant action, and realize aDC converter of low noise and high efficiency.

Also, this embodiment improves the flux use of a core of a transformer,reduces a ripple current of a smoothing capacitor in a secondarycircuitry of the transformer, and reduces the size of a DC converter.The embodiment realizes a proper cross regulation among multipleoutputs. The embodiment provides a rectangular output voltage from thesecondary side of a transformer, and therefore, is advantageous insynchronous rectification and can realize high efficiency with respectto a low output voltage.

Eighth Embodiment

FIG. 30 is a circuit diagram showing a DC converter according to aneighth embodiment. The DC converter according to the eighth embodimentshown in FIG. 30 differs from the DC converter according to the firstembodiment shown in FIG. 5 in that a reactor (fourth reactor) L1 isconnected between a diode D1 and a capacitor C4 and a diode D82 isconnected to a node between the diode D1 and a first end of the reactorL1 and to a second end of a secondary winding 5 b. The primary side ofthe transformer T is the same as that of the first embodiment, andtherefore, will not be explained.

A core of the transformer T is wound with a primary winding 5 a and thesecondary winding 5 b (the number of turns being n2) that is in the samephase as the primary winding 5 a. A first end of the secondary winding 5b is connected to the diode D1 (corresponding to a first rectifyingelement of the present invention). The node between the diode D1 and thefirst end of the reactor L1 and the second end of the secondary winding5 b are connected to the diode D82 (corresponding to a second rectifyingelement of the present invention). The diodes D1 and D82 form arectifying circuit. A second end of the reactor L1 and the second end ofthe secondary winding 5 b are connected to a capacitor C4 (correspondingto a smoothing circuit of the present invention). The capacitor C4smoothes a voltage of the reactor L1 and provides a DC output to a loadRL.

Operation of the DC converter according to the eighth embodiment withthe above-mentioned configuration will be explained with reference toFIGS. 31 and 32. FIG. 31 is a timing chart showing signals at variousparts of the DC converter according to the eighth embodiment. FIG. 32 isa timing chart showing the details of the signals at the various partswhen a switch Q1 is turned on. The B-H characteristic of the transformerof the DC converter and the timing chart of a current of a saturablereactor of the DC converter are the same as those of the firstembodiment shown in FIGS. 9 and 10.

FIGS. 31 and 32 show a terminal voltage Q1 v of the switch Q1, a currentQ1 i passing through the switch Q1, a terminal voltage Q2 v of a switchQ2, a current Q2 i passing through the switch Q2, a current Idc1 ipassing through a power supply source Idc1, and a current SL1 i passingthrough the saturable reactor SL1.

At time t1 (corresponding to t11 to t12), the switch Q1 is turned on.Then, a current passes in order of Vdc1, 5 a, Q1, and Vdc1. At the sametime, the secondary winding 5 b of the transformer T generates a voltageto pass a current in order of 5 b, D1, L1, C4, and 5 b. When the switchQ1 is turned on, the saturable reactor SL1 passes the current SL1 i toaccumulate energy in an inductor of the saturable reactor SL1.

The current SL1 i changes, as shown in FIG. 10, to a current value a(negative value) at time t1, to a current value b (negative value) attime t1 b, to a current value c (zero) at time t13, and to a currentvalue d (positive value) at time t2. On the B-H curve shown in FIG. 9,magnetic flux changes in order of Ba, Bb, Bc, and Bd. Ba to Bg shown inFIG. 9 correspond to a to g shown in FIG. 10.

At time t2, the switch Q1 is turned off. Then, the energy accumulated inthe saturable reactor SL1 charges a capacitor C1. At this time, theinductance of the saturable reactor SL1 and the capacitor C1 form avoltage resonance to steeply increase the voltage Q1 v of the switch Q1.A current passes in order of L1, C4, D82, and L1 to supply a currentthrough the capacitor C4 to the load RL.

When the potential of the capacitor C1 becomes equal to the potential ofa capacitor C3, the discharge of energy of the saturable reactor SL1makes a diode D4 conductive to pass a diode current that charges thecapacitor C3. At this time, the switch Q2 is turned on so that theswitch Q2 becomes a zero-voltage switch. From time t2 to time t20, thecurrent SL1 i changes from the current value d (positive value) to acurrent value e (zero). On the B-H curve shown in FIG. 9, magnetic fluxchanges from Bd to Be.

Simultaneously with the energy discharge of the saturable reactor SL1,energy from a power supply source Idc1 is supplied to the capacitor C3,which is charged. Namely, the capacitor C3 cumulatively receives boththe energy from the power supply source Idc1 and the energy from thesaturable reactor SL1. Upon the completion of the energy discharge ofthe saturable reactor SL1 and the energy discharge of the power supplysource Idc1, the charging of the capacitor C3 stops.

From time t20 to time t3, the energy accumulated in the capacitor C3flows in order of C3, Q2, SL1, and C3 to reset the magnetic flux of thesaturable reactor SL1. The magnetic flux of the transformer T that isconnected in parallel with the saturable reactor SL1 similarly changes.

From time t20 to time t3, the energy accumulated in the capacitor C3 isreturned to the saturable reactor SL1, and therefore, the current SL1 ipassed to the saturable reactor SL1 becomes negative as shown in FIG.10. Namely, the current SL1 i changes from the current value e (zero) toa current value f (negative value) in the period from t20 to t2 a. Onthe B-H curve shown in FIG. 9, magnetic flux changes from Be to Bf. Anarea S from time t2 to time t20 is equal to an area S from time t20 totime t2 a. The area S corresponds to the energy of the saturable reactorSL1 accumulated in the capacitor C3.

From time t2 a to time t3, the current SL1 i changes from the currentvalue f (negative value) to a current value g (negative value). On theB-H curve shown in FIG. 9, magnetic flux changes from Bf to Bg. An areafrom time t2 a to time t3 corresponds to the energy of the power supplysource Idc1 accumulated in the capacitor C3.

Namely, the energy accumulated in the capacitor C3 is the sum of theenergy of the saturable reactor SL1 and the energy of the power supplysource Idc1, and therefore, the current SL1 i is increased by the energysupplied from the power supply source Idc1 at the time of resetting. Asa result, magnetic flux moves to the third quadrant to reach thesaturation region (Bf to Bg) to increase the current SL1 i, whichbecomes the maximum at time t3 (similar to time t1). The current SL1 iincreases to a saturation current of the saturable reactor SL1 justbefore the end of an ON period of the switch Q2.

At time t3, the current Q2 i of the switch Q2 reaches the maximum. Atthis time, the switch Q2 is turned off to steeply discharge thecapacitor C1, which quickly becomes zero. At this time, the switch Q1 isturned on to achieve zero-voltage switching.

FIG. 33 is a circuit diagram showing the details of the DC converteraccording to the eighth embodiment. The eighth embodiment shown in FIG.33 forms the power supply source Idc1 from a series circuit composed ofa reactor L2 and a diode D6.

According to this embodiment, the reactor L2 accumulates energy when theswitch Q1 is turned on, and when the switch Q1 is turned off, the energyaccumulated in the reactor L2 is supplied to the capacitor C3 to chargethe capacitor C3. The power supply source Idc1 shown in FIG. 33 isappropriate for light load.

First Modification

FIG. 34 is a circuit diagram showing a first modification of the DCconverter according to the eighth embodiment. The first modificationshown in FIG. 34 forms the power supply source Idc1 from a reactor L3that is connected in series with the primary winding 5 a of thetransformer T.

According to the first modification, the switch Q1 is turned on to passa current through the reactor L3 and accumulate energy in the reactorL3. When the switch Q1 is turned off, the energy is discharged in orderof L3, 5 a (SL1), D4, C3, and L3. The energy is partly supplied throughthe secondary winding 5 b of the transformer T to the load RL and ispartly supplied to the capacitor C3 to charge the capacitor C3. Thepower supply source Idc1 shown in FIG. 34 is appropriate for heavy load.

Second Modification

FIG. 35 is a circuit diagram showing a second modification of the DCconverter according to the eighth embodiment. The second modificationshown in FIG. 35 combines the reactor L2 and diode D6 serving as thepower supply source Idc1 of FIG. 33 with the reactor L3 serving as thepower supply source Idc1 of FIG. 34, to cope with light load and heavyload.

The reactor L3 may be replaced with a leakage inductor of thetransformer T. The saturable reactor SL1 may be replaced with anexcitation inductance of the transformer T if the transformer employs acore having a good saturation characteristic. This circuit can controlan output voltage with a fixed switching frequency to perform PWMcontrol, thereby easily coping with a broadcasting interference and thelike.

Ninth Embodiment

A DC converter according to a ninth embodiment employs a synchronousrectifier in a secondary circuitry of a transformer. The transformeroutputs a rectangular waveform, and the embodiment increases aconduction ratio for synchronous rectification, to reduce a rectifierloss for a low output voltage and improve rectification efficiency. FIG.36 is a circuit diagram showing the DC converter according to the ninthembodiment.

The DC converter shown in FIG. 36 is the same as the DC converteraccording to the first modification of the eighth embodiment shown inFIG. 34 except the structure of the secondary circuitry of thetransformer T. Accordingly, the same parts are represented with the samereference marks, and only the structure of the secondary circuitry ofthe transformer T will be explained.

Both ends of a secondary winding 5 b of the transformer T are connectedto a switch Q3 made of a FET and a switch Q4 made of a FET, the switchesQ3 and Q4 being connected in series. A first end (marked with a dot) ofthe secondary winding 5 b of the transformer T is connected to a gate ofthe switch Q3, and a second end of the secondary winding 5 b of thetransformer T is connected to a gate of the switch Q4. The switch Q3 isconnected in parallel with a diode D1, and the switch Q4 is connected inparallel with a diode D82. These elements form a synchronous rectifyingcircuit.

Both ends of the switch Q4 are connected in series with a reactor L1 anda capacitor C4. This rectifying/smoothing circuit rectifies and smoothesa voltage (on/off-controlled pulse voltage) induced on the secondarywinding 5 b of the transformer T and provides a DC output to a load RL.

A control circuit 10 turns on and off switches Q1 and Q2 alternately.When an output voltage of the load RL becomes equal to or greater than areference voltage, the ON-width of a pulse applied to the switch Q1 isnarrowed and the ON-width of a pulse applied to the switch Q2 iswidened. Namely, when an output voltage of the load RL becomes equal toor greater than the reference voltage, the ON-width of a pulse appliedto the switch Q1 is narrowed to maintain a constant output voltage.

Operation of the DC converter according to the ninth embodiment with theabove-mentioned configuration will be explained with reference to atiming chart shown in FIG. 37. In FIG. 37, Q1 v is a terminal(drain-source) voltage of the switch Q1, Q1 i is a current (draincurrent) passing through the switch Q1, Q2 v is a terminal voltage ofthe switch Q2, Q2 i is a current passing through the switch Q2, Q3 i isa current passing through the switch Q3, Q4 i is a current passingthrough the switch Q4, SL1 i is a current passing through a saturablereactor SL1, and VT is a terminal voltage of the secondary winding 5 bof the transformer T.

In period T1 (corresponding to time t0 to t1 in FIG. 31 and time t2 totime t3 in FIG. 31), the switch Q1 is OFF and the switch Q2 is ON. As aresult, the switch Q2 passes a current and the switch Q1 passes nocurrent. At this time, the primary winding 5 a of the transformer Tgenerates a counter electromotive force (negative on the side markedwith a dot of the primary winding 5 a and positive on the other side).Due to the counter electromotive force, the secondary winding 5 bgenerates voltage (negative on the side marked with a dot of thesecondary winding 5 b and positive on the other side). As a result, thepositive voltage is applied to the gate of the switch Q4 to turn on theswitch Q4, and the negative voltage is applied to the gate of the switchQ3 to turn off the switch Q3. A current is passed in order of L1, C4,Q4, and L1, and energy in the reactor L1 is supplied to the load RL.

In a period from T2 to T4 (corresponding to time t1 in FIG. 31), theswitch Q2 is changed from the ON state to an OFF state, and the switchQ1 is changed from the OFF state to an ON state. Accordingly, theinductance of a reactor L3, the saturation inductance of the saturablereactor SL1, and a capacitor C1 resonate. The resonance decreases thevoltage of the switch Q1 and increases the voltage of the switch Q2(period T2). When the switch Q1 drops close to a zero voltage (periodT3), the switch Q1 is turned on, and a current is passed through theswitch Q1 (period T4).

In period T5 (corresponding to time t1 to time t2 in FIG. 31), theswitch Q1 is ON and the switch Q2 is OFF. At this time, a DC powersource Vdc1 supplies a current through the primary winding 5 a of thetransformer T to the switch Q1 to accumulate energy in the primarywinding 5 a (positive on the side marked with a dot of the primarywinding 5 a and negative on the other side). This energy generatesvoltage on the secondary winding 5 b (positive on the side marked with adot of the secondary winding 5 b and negative on the other side).Accordingly, the positive voltage is applied to the gate of the switchQ3 to turn on the switch Q3, and the negative voltage is applied to thegate of the switch Q4 to turn off the switch Q4. Then, a current ispassed in order of 5 b, L1, C4, Q3, and 5 b to supply DC power to theload RL. When the switch Q1 is turned on, the saturable reactor SL1passes the current SL1 i to accumulate energy in the inductor of thesaturable reactor SL1.

In period T6 (corresponding to time t2 of FIG. 31), the switch Q1 ischanged from the ON state to an OFF state. In the period T6, theinductance of the reactor L3, the inductance of the saturable reactorSL1, and the resonant capacitor C1 resonate. The resonance steeplyincreases the voltage of the switch Q1.

In period T7 (corresponding to time t2 of FIG. 31), a diode D4 turns onafter the switch Q1 is turned off, and the diode D4 passes a current. Asa result, energy in the saturable reactor SL1 and energy in the reactorL3 are accumulated through the diode D4 in the snubber capacitor C3.During the ON period of the diode D4, the switch Q2 is turned on so thatthe switch Q2 functions as a zero-voltage switch.

In this way, the DC converter according to the ninth embodiment providesthe effect of the eighth embodiment. In addition, the ninth embodimentemploys the synchronous rectifier in the secondary circuitry of thetransformer T. The transformer outputs a rectangular waveform, which isapplied to the gate of the synchronous rectifying element to make itconductive substantially during the whole period. Then, no currentpasses through the parallel-connected diode, and therefore, therectification is carried out without loss. This is effective for a lowoutput voltage such as 5 V or 3.3 V.

1. A DC converter comprising: a first series circuit being connected inparallel with a DC power source and including a primary winding of atransformer and a first switch that are connected in series; a firstreturn circuit being connected to the first series circuit and includinga second switch and a snubber capacitor that are connected in series andconfigured to return energy accumulated in a saturable reactor; thesaturable reactor connected in parallel with the primary winding of thetransformer and configured to operate in a saturation region when thesecond switch is in an ON state; a rectifying/smoothing circuitconnected in parallel with a secondary winding of the transformer andincluding a rectifying element and a smoothing element; and a controlcircuit to turn on and off the first and second switches alternately. 2.The DC converter of claim 1 further comprising: a power supply source toaccumulate power when the first switch is ON and supply the power to thesnubber capacitor when the first switch is OFF, the first return circuitbeing connected in parallel with any one of the first switch and primarywinding, the control circuit turning off the second switch when acurrent to the second switch increases.
 3. The DC converter of claim 2,wherein the saturable reactor is realized by using the saturationcharacteristic of a core of the transformer.
 4. The DC converter ofclaim 2, wherein the power supply source comprises a second seriescircuit connected to a first end of the DC power source and a nodebetween the first switch and the second switch and including a firstreactor and a diode that are connected in series.
 5. The DC converter ofclaim 2, wherein the power supply source comprises a second reactorconnected in series between the DC power source and the primary windingof the transformer.
 6. The DC converter of claim 5, wherein the secondreactor comprises a leakage inductor of the transformer.
 7. The DCconverter of claim 4, wherein the secondary winding of the transformercomprises a plurality of secondary windings that are wound around a coreof the transformer and are separated away from each other, each of thesecondary windings being provided with the rectifying/smoothing circuithaving the rectifying element and smoothing element.
 8. The DC converterof claim 7, wherein the primary winding of the transformer is looselycoupled with each of the secondary windings, and the secondary windingsare tightly coupled with each other.
 9. The DC converter of claim 2,wherein a magnetic path of a core of the transformer is locally providedwith a cross-sectional-area-reduced part.
 10. The DC converter of claim2, wherein: the DC power source comprises an AC power source and aninput rectifying circuit connected to the AC power source to rectify anAC voltage; a series circuit is connected between a first outputterminal and a second output terminal of the input rectifying circuit,the series circuit comprising an input smoothing capacitor and a rushcurrent limiting resistor that is connected in series with the inputsmoothing capacitor, to reduce a rush current of the input smoothingcapacitor when the AC power source is turned on; the first switchcomprises a normally-ON-type switch that is connected through theprimary winding of the transformer to the first output terminal of theinput rectifying circuit; and the control circuit turns off the firstswitch with a voltage generated by the rush current limiting resistorwhen the AC power source is turned on, and after the input smoothingcapacitor is charged, starts a switching operation to turn on and offthe first switch.
 11. The DC converter of claim 10, wherein thetransformer further includes an auxiliary winding, and the DC converterfurther comprises a normal operation power source to supply a voltagegenerated by the auxiliary winding of the transformer to the controlcircuit.
 12. The DC converter of claim 10, further comprising: asemiconductor switch connected in parallel with the rush currentlimiting resistor, the control circuit turning on the semiconductorswitch after starting the switching operation of the first switch. 13.The DC converter of claim 1, wherein: the first switch of the firstseries circuit is connected through a third reactor to the primarywinding; and a second return circuit is connected to the transformer, toreturn energy accumulated in the third reactor to the secondary side ofthe transformer.
 14. The DC converter of claim 13, wherein the secondreturn circuit includes an auxiliary transformer connected in serieswith the transformer, to return the energy accumulated in the thirdreactor when the first switch is ON to the secondary side when the firstswitch is OFF.
 15. The DC converter of claim 14, further comprising: apower supply source to accumulate power when the first switch is ON andsupply the power to the snubber capacitor when the first switch is OFF,the first return circuit being connected in parallel with any one of thefirst switch and primary winding, and the control circuit turning offthe second switch when a current to the second switch increases.
 16. TheDC converter of claim 14, wherein the third reactor comprises a leakageinductor between the primary winding and secondary winding of thetransformer that are loosely coupled around a core of the transformer,the primary winding of the transformer and the secondary winding of theauxiliary transformer being wound around the core of the transformer andbeing tightly coupled with each other.
 17. The DC converter of claim 13,wherein the saturable reactor is realized by the saturationcharacteristic of a core of the transformer.
 18. The DC converter ofclaim 15, wherein the power supply source comprises a second seriescircuit connected to a first end of the DC power source and a nodebetween the first switch and the second switch and including a firstreactor and a diode that are connected in series.
 19. The DC converterof claim 15, wherein the power supply source comprises a second reactorconnected in series with the primary winding of the transformer.
 20. TheDC converter of claim 19, wherein the second reactor comprises a leakageinductor of the transformer.
 21. The DC converter of claim 13, whereinat least one tertiary winding is wound around a core of the transformerand is loosely coupled with the primary winding of the transformer, andeach of the tertiary windings is provided with the rectifying/smoothingcircuit having the rectifying element and smoothing element.
 22. The DCconverter of claim 13, wherein a magnetic path of a core of thetransformer is locally provided with a cross-sectional-area-reducedpart.
 23. The DC converter of claim 14, wherein the rectifying/smoothingcircuit further comprises: a third switch connected to a node between afirst end of the secondary winding of the transformer and a first end ofa secondary winding of the auxiliary transformer and to a first end ofthe smoothing element, a control terminal of the third switch beingconnected to a second end of the secondary winding of the auxiliarytransformer; and a fourth switch connected to the second end of thesecondary winding of the auxiliary transformer and the first end of thesmoothing element, a control terminal of the fourth switch beingconnected to the first end of the secondary winding of the auxiliarytransformer.
 24. The DC converter of claim 13, wherein: the DC powersource comprises an AC power source and an input rectifying circuitconnected to the AC power source to rectify an AC voltage; a seriescircuit is connected between a first output terminal and a second outputterminal of the input rectifying circuit, the series circuit comprisingan input smoothing capacitor and a rush current limiting resistor thatis connected in series with the input smoothing capacitor, to reduce arush current of the input smoothing capacitor when the AC power sourceis turned on; the first switch comprises a normally-ON-type switch thatis connected through the primary winding of the transformer to the firstoutput terminal of the input rectifying circuit; and the control circuitturns off the first switch with a voltage generated by the rush currentlimiting resistor when the AC power source is turned on, and after theinput smoothing capacitor is charged, starts a switching operation toturn on and off the first switch.
 25. The DC converter of claim 24,wherein the transformer further includes an auxiliary winding, and theDC converter further comprises a normal operation power source to supplya voltage generated by the auxiliary winding of the transformer to thecontrol circuit.
 26. The DC converter of claim 24, further comprising: asemiconductor switch connected in parallel with the rush currentlimiting resistor, the control circuit turning on the semiconductorswitch after starting the switching operation of the first switch. 27.The DC converter of claim 1, further comprising: a power supply sourceto accumulate power when the first switch is ON and supplies the powerto the snubber capacitor when the first switch is OFF, the first returncircuit being connected in parallel with any one of the first switch andprimary winding, the rectifying/smoothing circuit including a secondrectifying element connected in parallel with the secondary winding ofthe transformer through the rectifying element and a fourth reactorconnected between the rectifying element and the smoothing element, thecontrol circuit turning off the second switch when a current to thesecond switch increases.
 28. The DC converter of claim 1, furthercomprising: a power supply source to accumulate power when the firstswitch is ON and supplies the power to the snubber capacitor when thefirst switch is OFF, the first return circuit being connected inparallel with any one of the first switch and primary winding, therectifying/smoothing circuit including: a fourth reactor connectedbetween the smoothing element and the secondary winding of thetransformer; a third switch connected in parallel with the rectifyingelement and having a control terminal connected to a second end of thesecondary winding and a fourth switch connected in parallel with aseries circuit of the third switch and secondary winding and having acontrol terminal connected to a first end of the secondary winding; anda second rectifying element connected in parallel with the secondarywinding of the transformer through the third switch, the control circuitturning off the second switch when a current to the second switchincreases.
 29. The DC converter of claim 27, wherein the saturablereactor is realized by using the saturation characteristic of a core ofthe transformer.
 30. The DC converter of claim 27, wherein the powersupply source comprises a second series circuit connected to a first endof the DC power source and a node between the first switch and thesecond switch and including a first reactor and a diode that areconnected in series.
 31. The DC converter of claim 27, wherein the powersupply source comprises a second reactor connected in series between theDC power source and the primary winding of the transformer.
 32. The DCconverter of claim 31, wherein the second reactor comprises a leakageinductor of the transformer.
 33. The DC converter of claim 27, wherein amagnetic path of a core of the transformer is locally provided with across-sectional-area-reduced part.
 34. The DC converter of claim 1,wherein the control circuit turns on the first switch within apredetermined period from the time when the voltage of the first switchbecomes zero due to resonance between a resonant capacitor connected inparallel with the first switch and the saturation inductance of thesaturable reactor.
 35. The DC converter of claim 5, wherein thesecondary winding of the transformer comprises a plurality of secondarywindings that are wound around a core of the transformer and areseparated away from each other, each of the secondary windings beingprovided with the rectifying/smoothing circuit having the rectifyingelement and smoothing element.
 36. The DC converter of claim 35, whereinthe primary winding of the transformer is loosely coupled with each ofthe secondary windings, and the secondary windings are tightly coupledwith each other.
 37. The DC converter of claim 28, wherein the saturablereactor is realized by using the saturation characteristic of a core ofthe transformer.
 38. The DC converter of claim 28, wherein the powersupply source comprises a second series circuit connected to a first endof the DC power source and a node between the first switch and thesecond switch and including a first reactor and a diode that areconnected in series.
 39. The DC converter of claim 28, wherein the powersupply source comprises a second reactor connected in series between theDC power source and the primary winding of the transformer.
 40. The DCconverter of claim 39, wherein the second reactor comprises a leakageinductor of the transformer.
 41. The DC converter of claim 28, wherein amagnetic path of a core of the transformer is locally provided with across-sectional-area-reduced part.